1. Field of the Invention
The present invention relates to an apparatus for generating pulse-width data for expressing a multiplicity of gray scales to supply the pulse-width data to a printing engine of a toner-transfer-type printing apparatus, such as a laser printer or a copying machine.
2. Description of the Related Art
A toner-transfer-type printing apparatus is able to print a multi-gray-scale image by adjusting the pulse width of a drive pulse signal for driving drawing laser in a printing engine because the quantity of toner for each pixel to be allowed to adhere can be changed. Since the minimum size of toner particles however is limited, the number of gray scale levels, which can be expressed, is reduced in inverse proportion to the resolution of an image required to be printed. An assumption is made that image data having a resolution of 600 dpi and 256 gray scale levels is created on a personal computer and image data above is printed by a laser printer having a printing resolution of 600 dpi. One pixel having a resolution of 600 dpi is in the form of a square region, one side of which is {fraction (1/600)} inch. The number of toner particles allowed to adhere to the square region is limited to about 20 to 30. Therefore, the image printed by the laser printer is able to reproduce at most 20 to 30 gray scale levels.
As described above, the conventional toner-transfer-type printing apparatus suffers from a problem in that the apparatus cannot reproduce a number of gray scale levels on a print output therefrom even if image data expressing the number of gray scale levels exceeding the number of toner particles allowed to adhere to one pixel region is supplied.
Accordingly, an object of the present invention is to enable a toner-transfer-type printing apparatus to reproduce, on an output image therefrom, a number of gray scale levels which is larger than the number of toner particles allowed to adhere to one pixel region, and more specifically to print an output image with which the sense of sight of a human being recognizes a fact that such a large number of gray scale levels has been reproduced.
To achieve the above-mentioned object, according to the present invention, there is provided a pulse-width data generating apparatus for use in a toner-transfer-type printing apparatus. The apparatus according to the present invention comprises pulse-width generating means which receives the image data to generate pulse-width data of each pixel corresponding to coordinates of each pixel in a Nxc3x97M pixel matrix (N and M are positive integers) virtually tiled on the output image and the gray-scale data of each pixel in the image data; and position-data generating means which receives the pulse-width data to subject the pulse-width data of pixels on two sides of each pixel on the output image to a comparison to generate position data for controlling toner adhesion positions of each pixel based on a result of the comparison. Preferably, a memory having Nxc3x97M pixel memory regions corresponding to Nxc3x97M pixels in the Nxc3x97M pixel matrix on the output image is provided, and pulse-width data corresponding to values for the gray-scale data are stored in each of said pixel memory regions. When the coordinates of each pixel in said Nxc3x97M pixel matrix corresponding to the pixels in the image data and the gray-scale data of each pixel in the image data are provided to the memory, the pulse-width data corresponding to the gray-scale data are read out from said pixel memory region addressed by the coordinates.
The pulse-width data generating apparatus according to the present invention determines the pulse width data corresponding to the coordinates of the pixels in the Nxc3x97M matrix, and can change the pulse width data among the pixels in the Nxc3x97M matrix slightly. Therefore, it enables the overall Nxc3x97M matrix on the output image to reproduce the number of gray scale levels which is approximately Nxc3x97M times the number of gray scale levels which can be expressed by one pixel. In general, it is difficult for the sense of sight of a human being to distinguish pixels each having a high resolution of about 300 dpi from each other. When a printing engine for forming an output image having a high resolution of, for example, 600 dpi is employed, expression of a required gray scale with an appropriate size near the minimum limit for the sense of sight for a human being, such as 2xc3x972 pixel matrix (corresponding to 300 dpi) or 3xc3x973 pixel matrices (corresponding to 200 dpi), enables for the sense of sight for a human being to recognize as if the required number of the gray scale levels has been reproduced on the output image.
It is preferable that an inverted xcex3-characteristic for correcting a xcex3-characteristic of the printing engine is incorporated to pulse-width data in the memory. In the foregoing case, a structure in which the inverted xcex3-characteristics incorporated to pulse-width data are made to be slightly different from one another among the pixels in the Nxc3x97M matrix causes a precise inverted xcex3-characteristic to appear in pulse-width data of the overall Nxc3x97M matrices attributable to a principle like the above-mentioned principle with which the number of gray scale levels which can be expressed increases. As a result, precise xcex3-correction can be performed.
Further, the apparatus according to the present invention subjects pulse-width data items of pixels on the two sides of each pixel to a comparison, and generates position data of each pixel corresponding to a result of the comparison. This position data indicates a pixel of the pixels on the two sides of each pixel that has higher density. When the position at which a pulse signal to be supplied to the printing engine is generated is controlled in accordance with position data, adjustment can be performed in such a manner that the position at which toner for each pixel on the output image is allowed to adhere is approached to the pixel of the side pixels having higher density. The process for collecting the positions, at each of which toner is allowed to adhere to form pixels, to integrate dots has a conventional technique for improving the quality of an output image. However, the apparatus according to the present invention, arranged in such a manner that the toner adhesion position is automatically determined in accordance with pulse-width data for each pixel, enables a necessity of previously providing, to image data, position data for instructing the toner adhesion position to be eliminated. Thus, an advantage can be realized in that the capacity of the image memory for storing image data can be saved by the foregoing degree.